The localization in the guinea pig enteric nervous system (ENS) of monoamine oxidase (MAO) types A and B was investigated at the light and electron microscopic levels. Immunocytochemistry was used to visualize the enzyme protein and histochemistry was employed to study catalytic activity. Type specificity was achieved in histochemical studies by using deprenyl (0.5 μM) to inhibit MAO‐B or clorgyline (0.1 μM) to inhibit MAO‐A. The distribution of MAO‐B immunoreactivity in the ENS corresponded to that of the sites of MAO activity found histochemically to be inhibited by deprenyl, but not clorgyline. MAO‐B was observed to be the primary type of MAO found in the intrinsic elements of the ENS and was located in subsets of neurons in both submucosal and myenteric plexuses. MAO‐B was not demonstrated immunocytochemically or histochemically in enteric glia, nor, at the light microscopic level, was there significant MAO‐B activity or immunoreactivity in serotonin (5‐HT)‐immunoreactive neuronal cell bodies. In the submucosal plexus about 50% of the neurons expressed MAO‐B; these neurons also contained neuropeptide Y (NPY) and/or calcitonin gene related peptide (CGRP), but not substance P or vasoactive intestinal polypeptide (VIP). About 10% of myenteric neurons were intensely reactive for MAO‐B; again MAO‐B was co‐localized with NPY and/or CGRP. In contrast to intrinsic neurons, extrinsic CGRP‐immunoreactive nerve fibers contained no demonstrable MAO activity or immunoreactivity. Moreover, the sympathetic innervation, identified as varicose axons that degenerated after administration of 6‐hydroxydopamine, contained abundant MAO‐A, but no MAO‐B activity or immunoreactivity. It is concluded that MAO‐B is characteristic of a subset of intrinsic enteric neurons, while MAO‐A is confined to the sympathetic innervation, which is extrinsic. At the electron microscopic level individual cells varied greatly in their degree of immuno‐ or cytochemically demonstrable MAO‐B, which was most concentrated on the outer membranes of mitochondria. MAO‐B immunoreactivity (but not cytochemical activity) was found on mitochondria in some serotoninergic perikarya identified by the simultaneous radioautographic detection of the uptake of 3H‐5‐HT. Mitochondria in most serotoninergic axon terminals displayed both MAO‐B activity and immunoreactivity. Neurons receiving serotoninergic synapses often, but not invariably, contained MAO‐B. Inhibition of neither MAO‐B nor MAO‐A appeared to slow the disappearance of 3H‐5‐HT loaded into enteric neurons significantly, even when intraneuronal storage of 5‐HT was inhibited with tetrabenazine. Alternative pathways for 5‐HT catabolism, such as glucuronidation, which do not occur in the brain, may be available to enteric serotoninergic neurons. The role of MAO‐B in enteric neurons that are not monoaminergic but peptidergic remains to be determined.
All Science Journal Classification (ASJC) codes
- calcitonin gene related peptide
- myenteric plexus
- neuropeptide Y
- submucosal plexus
- substance P
- vasoactive intestinal polypeptide